Differentiation of rhizosphere fungal assemblages by host ploidy level in mixed-ploidy Larrea tridentata populations

Polyploidy--whole genome duplication--is common in plants. Studies over the last several decades have documented numerous mixed-ploidy populations. Whether arising via recurrent whole genome duplication events within a population, or from secondary contact, the persistence of mixed populations depends on the ability of the minority cytotype to overcome the negative frequency dependent effects of outcrossing with other ploidies, known as Minority Cytotype Exclusion. One mechanism of overcoming Minority Cytotype Exclusion is microbially-mediated niche differentiation (MMND), wherein cytotypes occupy different niches via interactions with different sets of microbes. Inherently cryptic, MMND is underexplored in polyploid plant populations. Here, we search for evidence of MMND in creosotebush (Larrea tridentata), a dominant desert shrub of the southwestern U.S. and northern Mexico. We sequenced fungi from rhizosphere soils of diploid, autotetraploid, and autohexaploid plants growing in two naturally-occurring mixed-cytotype populations. Within populations, we found substantial fungal assemblage overlap across host plant cytotypes. However, using indicator species analysis, we identified some fungi that are differentiated by host plant cytotype, satisfying a precondition for MMND. Future study is needed to determine the degree of niche differentiation conferred, if any, and whether the identified fungi play a role in the long-term persistence of multiple cytotypes within populations.